Piezoelectric materials are materials that can generate charge and provide voltage when placed under mechanical stress (“piezoelectric effect”), and conversely can be deformed under an applied electrical field (the “converse piezoelectric effect”). In general, piezoelectric materials exhibit these two reciprocal effects characterized by the piezoelectric constants “d” and “g”, respectively. Commonly made piezoelectric constant measurements include the “D33” piezoelectric charge coefficient (or “d33”, in units of picocoulombs per Newton, pC/N) and the “G33” piezoelectric voltage coefficient (or “g33”, in units of pm/V). These piezoelectric coefficients express the piezoelectric response of a material when stress or voltage is applied parallel to the crystallographic c-axis of the piezoelectric material or parallel to the poling direction of the piezoelectric layer and the material response is also measured along to the crystallographic c-axis of the piezoelectric material or along the poling direction of the piezoelectric layer.
Piezoelectric devices have many applications such as for high voltage generation (for example, as gas lighters producing a spark), microactuators, microbalances, acoustic generators (including ultrasound generators), vibration sensors, and other devices known in the art.
A common piezoelectric material is lead zirconate titanate (PZT) but as one might suspect, such material raises both environmental and public health concerns relating to both production and use because of the presence of lead. U.S. Pat. No. 6,469,421 (Wakabayashi et al.) describes the use of a layer of ultra-fine particles of the same material in a PZT piezoelectric layer. While lead zirconate titanate performs well, it has proven difficult in recent years to find materials with comparable performance so that PZT can be replaced. It is also difficult to make either thick or thin film piezoelectric devices of high sensitivity, and thus, efforts are being undertaken to find improved materials without the presence of lead.
A study of reciprocity effects using a layer of loose tin particles on the surface of a piezoelectric device (transducer) comprised of poly (vinylidene fluoride) (PVDF) that is a piezoelectric material is reported by Faust and Lakes in Physica Scripta 90 085807 (2015). While the work of these investigators suggests that non-uniform stress fields may change the piezoelectric charge constant d33, their approach was to use a loose powder on the surface of the piezoelectric material. This loose powder can be readily dislodged from the piezoelectric layer and any change in d33 is thereby diminished or lost entirely. Thus, the transducer they proposed is mechanically unstable and impractical for any industrial use.
Flexoelectric piezoelectric composites are described in U.S. Pat. No. 7,791,248 (Cross et al.) as having unique shapes in order to modify the electrical signals obtained from applied forces. The materials described in this patent are rigid bodies and the teaching does not lead itself to using flexible materials. In addition, flexoelectric effects are generally smaller than piezoelectric effects, as noted by Krichen and Sharma in the J. Appl. Mech. 83, 030801-1 to 030801-5 (2016).
It is recognized by those skilled in the art of piezoelectric materials that piezoelectric polymers (such as PVDF), in general, have lower piezoelectric coefficients than crystalline inorganic piezoelectric materials such as PZT and that the lower piezoelectric coefficients of piezoelectric polymers have hampered the development of uses for these materials. Many uses of piezoelectric materials will benefit from any method for improving the piezoelectric constants (for example, the d33 piezoelectric charge coefficients) of the material. Thus, there is a need for improved piezoelectric materials, methods of improving the piezoelectric coefficient of piezoelectric materials, improved methods of packaging piezoelectric materials to enhance the piezoelectric coefficients of the piezoelectric materials, and improved devices containing piezoelectric materials with enhanced piezoelectric coefficients.
Particularly, it is desirable to increase the magnitude of the fundamental piezoelectric coefficients d33, d31, or any other piezoelectric coefficients in order to improve the overall performance of devices that utilize this piezoelectric coefficient for operation.